The basic unit of semiconductor integrated circuit devices is a metal oxide semiconductor field effect transistor (MOSFET) device which includes a thin dielectric material, commonly a thermally grown oxide, which functions as a gate dielectric for transistors which are formed within the semiconductor substrate. The gate dielectric material is typically formed on the semiconductor substrate over a region within the substrate which will serve as a channel region of a transistor. The transistors function when the channel is formed in the semiconductor substrate beneath the gate dielectric in response to a voltage being applied to a gate electrode disposed atop the gate dielectric film. The quality and integrity of the gate dielectric film is critical to the functionality of the transistor devices. It is important, therefore, to suppress the migration of any undesired dopant species into the gate dielectric film.
Polycrystalline silicon (polysilicon) films are commonly used as interconnect materials and as gate electrodes for transistors in semiconductor integrated circuits. Impurity species are commonly added to polysilicon films to reduce sheet resistance. The addition of impurities is also referred to as "doping". Polycrystalline silicon is commonly "N-type" polycrystalline silicon. By "N-type" polysilicon material, it is meant that an N-type impurity is introduced into the polycrystalline silicon film. However, when it is desired to produce a device which operates at relatively low voltages, both N-type and P-type impurity regions are necessary. In CMOS devices, N-type polysilicon is used for P-channel devices and P-type polysilicon is used for N-channel devices.
A commonly used, and preferred P-type dopant impurity within the semiconductor industry is boron. Boron is very effective in lowering the sheet resistance of a film into which it is incorporated. When boron is used as an impurity dopant within a polycrystalline silicon film, it is of critical significance to maintain the boron within the polycrystalline silicon film, and especially to suppress the migration of the boron dopant impurity into the gate dielectric film which forms part of the transistor, and also laterally into adjacent N-type doped regions. Boron is a very active species, and diffuses quite rapidly into adjacent materials, especially oxide materials, during the subsequent high temperature processing used to manufacture semiconductor devices.
Boron may diffuse laterally or vertically within the semiconductor structure. In addition to boron diffusing into the gate dielectric material, the boron can further diffuse through the gate dielectric material and into the channel region of a transistor formed below the gate dielectric region. When this occurs, device functionality can be destroyed. It is thus of increased significance to suppress the vertical and lateral diffusion of boron from the polysilicon interconnect and gate structures
In addition to diffusing into and through gate dielectric materials, boron is also susceptible to diffusing into nearby N-type impurity regions thereby altering the doping characteristics of the N-type regions. For example, after a boron doped polysilicon film is formed, an N-type impurity region may be formed within or adjacent the boron doped polysilicon film. An N-type impurity region may be formed within a P-type region by counterdoping the P-type region with a sufficient dose of N-type impurities so as to overcome the P-type character of the material being doped. Phosphorus and arsenic are common examples of N-type dopant impurities.
After the N-type region is formed, boron may diffuse from the P-type region into the N-type region during subsequent high temperature processing, thereby lessening or negating the effect of the N-type impurity dopants within the N-type region, depending on the original dopant concentration of the N-type impurity forming the N-type regions. In this manner, the N-type doping characteristic of the N-type region is compromised by the addition of the P-type boron impurities to the region by way of diffusion. Subsequent processing may be required to adjust this compromised doping characteristic and to restore the original N-type dopant concentration within the N-type region.
Three desirable qualities for a doped silicon film are a low sheet resistance, a uniform dopant distribution within the film, and a uniform grain structure with relatively large grain sizes. When a silicon film is a polycrystalline silicon film as deposited, it normally contains grains which vary significantly in orientation and size. When a polysilicon film is doped, either in-situ or by way of a subsequent doping process such as ion implantation, the dopant species tend to segregate into the grain boundaries of the film. Therefore, when the grains and grain boundaries are not uniform within the film, the dopant uniformity within the film, as well, cannot be uniform.
Because of the above problems associated with the lateral and vertical diffusion of boron, there is a demonstrated need in the art to provide a process and structure which suppresses boron diffusion from P-type polysilicon regions, through polysilicon conductors, and into N-type regions when subsequent processing takes place at elevated temperatures which may cause boron diffusion.
With today's advancing technology, and as device sizes continue to shrink, it is desirable to produce tightly packed adjacent N-type and P-type regions within the same original film and which are resistant to cross diffusion. It is further desirable to produce a polysilicon film which is doped with boron, and which contains large grain sizes within an ordered grain structure having a uniform boron dopant distribution therein. It is also desired to produce the above features using a simplified process sequence with a reduced number of processing steps, which results in reduced production times and costs.